Numerous proposals have been made for the fabrication of precast panels for use in the construction of buildings. Such precast panels are attractive in that they enable the walls of a building to be covered in a more economical manner, and in a shorter space of time than with conventional wall coverings such as bricks, stone, and the like.
In addition, precast panels enable buildings to be covered in with a wide variety of different decorative surfaces and decorative effects moulded into the precast panels.
In addition, since the precast panels can be prefabricated away from the building site, preferably at, for example, a factory, factory labour can be used usually at higher production rates and at lower wage rates than on-site construction labour. In addition, a greater accuracy is possible in the production of such panels, so that the end result in the finished building is both aesthetically pleasing, and is also efficient and effective.
One of the most popular forms of precast panel is a solid panel of concrete, into which is embedded one or more layers of reinforcing steel mesh. Usually such precast panels are at least three inches or more in thickness.
Such panels are of extreme weight. In addition, they have a very low R value. That is to say, the thermal insulation offered by a precast concrete panel is very small. Variations in exterior temperature are rapidly transmitted through to the interior of the building.
Consequently, in this form of construction using precast concrete panels, the panels are customarily supported on the building fabric, which may be either concrete columns, or steel columns, and then interior walls complete with insulation will usually be installed, so as to provide a stable controllable climate within the building.
In addition, because of the extreme weight of the panels, the anchoring system whereby they are anchored to the building must be very carefully designed to withstand all possible stresses due to climate, length of time of use, and also, if erected in earthquake zones., they must be capable of withstanding a certain degree of seismic shock.
All of these factors are very well known and understood by construction engineers generally.
Notwithstanding the obvious disadvantages of such solid precast panels however, they have remained in general use for very many years, in spite of numerous attempts to replace them with a more economical alternative.
For example, there is disclosed in U.S. Pat. No. 4,602,467, dated Jul. 19, 1986, inventor H. Schilger, a form of precast concrete panel, which is reinforced with simple generally C-shaped channel sections of steel. Edge portions of the C sections are formed in various different ways for embedment in the concrete. Using this system, it is said that a substantial reduction in the thickness of the concrete is possible. A similar system is illustrated in Canadian Letters Patent 1,264,957. The steel C sections are said to add rigidity to the thin shell of concrete which forms the exterior panel.
Using this type of system, buildings were in fact constructed, in which the building thermal insulation was placed between these steel C sections. The interior wall surface of the building, typically being some form of dry wall panels, was attached directly to the interior edges of the steel C sections.
Numerous examples of earlier similar proposals are illustrated in the art, listed in that U.S. patent.
There are however numerous problems resulting from the use of this type of proposal.
In the first place, concrete and steel have differential rates of expansion and contraction. Consequently, when exposed to extremes of heat and cold the steel will tend to extend or contract along its length, a distance greater than that of the concrete. Consequently, over time, there will be a gradual working or movement between the steel and the concrete, which may loosen the bond between the steel and the concrete.
An additional problem is the fact that in such earlier proposals, the edge portion of the steel which was embedded in the concrete, constituted a "break line" extending, usually vertically, along the panel at spaced intervals. Bearing in mind that it was intended with this type of thin shell panel to reduce the concrete thickness to less than two inches and, in some cases, one and a half inches was mentioned, the existence of such a continuous break line at regular intervals across the panel constituted possible fracture lines if the panels were exposed to unusual shocks.
While these particular disadvantages and hazards might not arise with any great frequency, a much more serious problem was the problem of heat transfer. The sheet metal reinforcement C section members, being embedded in the relatively thin exterior concrete panel of the building, acted as ideal heat transfer bridges transferring heat one way or the other depending upon the season, through the wall.
This was particularly noticeable in the colder seasons. In these seasons, when the ambient air outside the building is cold, and the interior of the building is heated, heat is transferred out through the wall, along the line of each of the C sections. This lost heat creates cold zones in the interior walls, along the lines of the channels. These cold zones, in turn, result in lines of condensation of moisture, which condenses out of the air and settles on the walls. The effect is generally known in the construction industry as "ghosting" on the walls, and is regarded as unacceptable under almost all building codes.
As a result, when using this system it is generally necessary to place a layer of insulation over the C sections, or some other form of thermal break was built into the construction of the wall, so that the interior wall of dry wall panels or the like, was kept completely out of contact with the C sections. However, this greatly increased the cost of the construction system, and as a result, tended to discourage builders from using the system.
One greatly improved form of construction panel overcoming a number of these problems is shown in U.S. Pat. No. 4,909,007, dated March 1990, inventor Ernest R. Bodnar.
In that Patent, there is disclosed a precast panel, reinforced with sheet metal stud members. The stud members are formed with diagonal struts at spaced intervals defining openings therebetween. In this way, the heat transfer bridge is reduced, leading to a reduced heat transfer path, and reducing if not completely eliminating the ghosting problem.
In the system, it was possible to attach the interior dry wall panels directly to the sub-members, thereby reducing the overall cost of the building system.
An additional advantage of this system was the fact that the edge portion of the stud which was formed to be embedded in the concrete was formed either as bent over tabs or as pierced holes, so that some of the concrete in the panel could flow through the holes or around the tabs, thereby reducing the degree of weakening of the stud. In addition, problems caused by differential rates of expansion and contraction were also reduced.
However, the stud disclosed in that patent involves the production of a stud with a relatively high degree of wastage of steel, caused by the cutting out of portions of the sheet metal between the struts. In addition, even though the embedded edge portion of the stud was formed with holes, portions of it were still continuous, and caused, to a minor degree, some of the problems encountered and described above.